=>Until July 2019, Venµs L2A products were processed at the Venµs Image Processing (VIP) center, with a specific format. From July 2019, the L2A are processed at MUSCATE with a slghtly different format which is described here. 

Unzip

When you download a Venµs L1C or L2A product from Theia website, you obtain a zip file.

unzip -o  VENUS-XS_20190428-173944-000_L2A_ARM_D_V2-15.zip

After unzipping, you get a folder with the same name as the zipfile, without ".zip".

L2A Format

The main product format contains a metadata file in xml format, the surface reflectances in two flavours, the atmospheric variables computed by MAJA, and a MASK folder.- the MTD_ALL.xml file contains the metadata from the image acquisition

- Instrument, date and time- projection and geographic coverage- Solar and viewing angles.
For the solar angles, you can use the values at the scene center.
<Solar_Angles><Useful_Image><Image_Center><Azimuth unit="deg">62.0585933294</Azimuth><Zenith unit="deg">34.1848602257</Zenith></Image_Center></Useful_Image>
</Solar_Angles>
For the viewing angles, you have to account for the fact that the angles differ depending on the spectral band.Venµs channels are regrouped per triplets and the viewing angles are provided for each triplet. For instance, for the third one :
<Viewing_Angles sn="3">
<Image_Center><Azimuth unit="deg">191.83414</Azimuth><Zenith unit="deg">26.282076</Zenith></Image_Center>
</Viewing_Angles>

 -  the surface reflectances are provided with two flavours. There is a file for each of Venµs 12 spectral bands (XX) hereafter :

  • the files which end with "FRE_BXX.TIF" (FRE for Flat REflectance) are also corrected for slope effect, which consists in suppressing the apparent reflectances variations due to the orientation of the slopes with regard to the sun. The corrected images look like if the land was flat.

Both types of files contain :

- surface reflectances for spectral band XX- coded in 16 bits signed integers: you have to divide per 10000 to obtain reflectances- No_Data value (outside the image), is -10000

- We also provide the values of the atmospheric variables computed by MACCS/MAJA, with a 5 m resolution (ATB_XS.tif)

  • The files contain two bands coded on 8 bits each :
    • The files contain two bands coded on 8 bits each :Band 1 is water vapour, you have to divide by 20 to obtain g/cm2  <VAP_Quantification_Value>0.05</VAP_Quantification_Value>Band 2 is the Aerosol Optical Thickness, you have to multiply by 0.005 to get the AOT. <AOT_Quantification_Value>0.005</AOT_Quantification_Value>
    The MASK directory provides :
  • A no_data mask, at full resolution (EDG_XS.tif)
  • A saturated pixel mask that comes from Level 1  (SAT_XS.tif)
    • first bit corresponds to first band
    • second bit corresponds to second band
    • ...
  • An interpolated pixel mask that comes from Level 1  (PIX_XS.tif). Some pixels in the blue bands are interpolated.
    • first bit corresponds to first band
    • second bit corresponds to second band
    • ...
  • A cloud and shadows mask is provided in the file CLD.DBL.TIF:
    bit 0 (1) : all clouds except the thinnest and all shadows
    bit 1 (2) : all clouds (except the thinnest)
    bit 2 (4) : cloud shadows cast by a detected cloud
    bit 3 (8) : cloud shadows cast by a cloud outside image
    bit 4 (16) : clouds detected via mono-temporal thresholds
    bit 5 (32) : clouds detected via multi-temporal thresholds
    bit 6 (64) : thinnest clouds
    bit 7 (128) : high clouds detected by stereoscopy

    • Example 1 : value 5 = 4+1 (00000101) :  bit 0 tells us a shadow or a cloud has been detected and bit 2 that it is a shadow (4),
    • Example 2 : value 35 =  32 + 2 +1 (00100011) tells us that it is a cloud (bits 1 and 2), detected by multi-temporal threshold (bit 5)
    • How to use the cloud mask

      If you want a very strict cloud and shadows mask, just test if cloud mask is greater than 0. This is our advice.If you are working in a region where you think MACCS/MAJA detects too many clouds, check that bit 1 is 1. (Mask & (00000001) == 1)

     

  • A geophysical mask MG2 :
  • bit 0 (1) : Water mask
    bit 1 (2) : All clouds (except the thinnests)
    bit 2 (4) : Snow Mask
    bit 3 (8) : all shadows ("OU" des bits 5 et 6 du masque de nuages)
    bit 4 (16) : Topographic shadows
    bit 5 (32) : Unseen pixels due to topography
    bit 6 (64) : Sun too low for a correct terrain correction
    bit 7 (128) : Sun direction tangent to slope (inaccurate terrain correction)
  • IAB : A mask of pixels for which the WaterVapour (bit 1) and Aerosol Optical Thickness (bit 2) was interpolated  (available at resolutions R1 or R2). However, in MAJA 3.2, there is an inversion of bit 2, which equals zero if interpolated)
  •   

    Laisser un commentaire

    Votre adresse de messagerie ne sera pas publiée. Les champs obligatoires sont indiqués avec *